SMALL PROJECTION SCREEN ABC
The intended use should always be analyzed before selecting the right projection screen. There are films that can be used universally and projection films whose area of application is clearly defined.
The "Small projection screen ABC" serves to highlight the most important points that need to be considered when selecting a projection screen and to explain the key terms relating to the projection screen.
ANSI lumen
ANSI lumen is a measure of the luminous flux of a projector. ANSI is the abbreviation for the American National Standards Institute, the American body for standardizing industrial procedures. The German counterpart is the Deutsches Institut für Normung e. V. (DIN). Both are members of the International Organization for Standardization (ISO). The ANSI lumen is the measured value for comparing the performance of projectors (beamers) with one another. The luminous flux is calculated from the luminous intensity (in lux) and the illuminated projection surface (in m²).
Front projection
Projection that is projected onto the front of a projection screen, onto the viewer's side, so to speak. The following Gerriets projection screens are suitable for front projection:
GAMMALUX®, GAMMALUX® MICRO, SCENE, SCENE SMOOTH, GREYSCREEN, OPERA® WHITE, OPERA® WHITE SMOOTH, OPERA® WHITE MICROPERFORATED, OPERA® HIGH GAIN, SILVERBLACK
Can be used on both sides
The following Gerriets projection screens are suitable for both front and rear projection:
OPERA® MILKY MATT, OPERA® LIGHT BLUE, SHOW®, STUDIO®, REVUE, EVEN, OPTIBLACK 2.2
Image format
The image format expresses the ratio of width to height of a displayed image.
- Square format 1:1
- Panorama format 2:1 / 3:1
- Slide format 3:2
- Video format 4:3
- Cinema or HDTV format 16:9
- WUXGA format 16:10
When buying a projection screen, it makes sense to consider the most commonly used format.
Screen size according to DIN 19045
As a guide, the width of the screen should be greater than 1/6 of the maximum viewing distance and the minimum distance to the screen should not be less than 1.5 times the screen width. The height of the screen depends on the preferred format, e.g. 4:3 or 16:9.
Screen types D and R
Screen type D (diffuse) stands for a front projection screen that reflects the incident light broadly and does not have a pronounced preferred direction of reflection.
Screen type R (rear projection) does not reflect the incident light but allows the light to pass through (transmission). These rear projection screens largely scatter the light passing through them and do not have a pronounced preferred direction of transmission. Front and rear projection screens are designated accordingly as D/R.
Type B projection screens with a crystal glass-coated surface or type S projection screens with a metallized surface are less common. These are projection screens with reflection in a preferred direction. These projection screens usually have a high luminance factor but a low scattering angle.
Color spectrogram and color rendering
Whether a uniform reflection/transmission is achieved across all color ranges can best be seen in a color spectrogram. The color spectrogram shows the reflection or transmission in the visible light range (approx. 380-780 nm) over the entire screen surface. The reflection or transmission behavior of the visible light of a projection surface in the different color ranges can be quite different. In contrast to the luminance factor, the color spectrogram measures the average value in all nanometer ranges over the entire screen, so that low reflection/transmission values do not necessarily indicate a low luminance factor. The true color rendering of a projection screen therefore has less to do with the degree or intensity of reflection (front projection) or transmission (rear projection) than with the uniformity in the different wavelengths. The more homogeneous the measured values and the more uniform the curve in the color spectrogram displayed, the better the true color reproduction of a projection screen.
The value 1 (or 100 %) shown in a color spectrogram is made up of the reflection, transmission and absorption of the projection screen.
The loss factor due to absorption is usually negligible, except in the case of poor quality projection screens.
The reflection or transmission factor is often confused with the luminance factor, but the measuring methods and the significance of the measured values are different.
Film size
Projection films of almost any size can be produced in a high-frequency welding shop. However, there is a physical limit to the height of soft PVC projection films, as the entire weight of the material places a static load on the upper area of a projection film and the material sags. For greater heights, the use of fabric-reinforced projection screens should be considered, but rear projections are not practical in this case. In principle, there is no restriction on the width, only the total weight of the projection surface should be taken into account. When choosing the size of a projection screen, attention should also be paid to a reasonable width-to-height ratio, as a soft PVC projection screen tends to waist if the height is greater than the width.
Film processing
High-frequency welding technology is the most modern, effective and high-quality welding process currently available on the market. Fine, almost invisible seams are created in several processing steps. This method can also be used to process lacquered films and other PVC films. Inflatable films with valve inserts are also possible, for example.
Durability
The durability of a screen depends less on the material properties than on the processing and the conditions on site. After several years of use, a projection screen may show some weak points due to storage and movement. Added to this are handling errors and thermal damage caused by storage and movement that is too cold. The biggest enemy of a projection screen is still performance in repertory houses, where the demands of a projection screen cannot usually be taken into account during assembly and dismantling. PVC that is immediately taken down into storage at playing temperatures of around 40 °C and cools down there when folded, sometimes to temperatures just above freezing in winter, will naturally suffer damage when it is put back into the trains.
Cold break
PVC changes its consistency depending on the temperature. PVC projection films become softer when warm and harder when cold. PVC projection films may only be processed and installed at normal room temperatures. If films are processed, installed or otherwise subjected to stress in ambient temperatures that are too cold, this can lead to so-called "cold breakage". This means irreversible damage to the film.
Contrast
In simple terms, contrast refers to the relationship between black and white. We speak of good image contrast when the viewer perceives black parts of the image as black and white parts of the image as white. To measure the contrast ratio, a rectangle with eight white and eight black fields is projected and the luminous flux reflected by the projection screen is measured using an NIT meter. The average value of the eight white and eight black measuring points is determined and set in relation to each other. This value describes the contrast ratio. The following values are recommended for projections under normal conference room conditions (ambient light at approx. 200 lux): a contrast ratio between 5:1 and 10:1 is considered poor, while a contrast ratio of 20:1 or higher is considered good quality.
Luminance diagram
The luminance diagram shows the luminance factor at the respective viewing angle. The flatter the curve, the more uniform the luminance distribution of the projection screen. The steeper the curve, the more the projection screen tends towards a hot spot (bright spot/circle in the center of the image) and is not suitable for certain applications (e.g. soft-edge projections).
Luminance factor
The luminance factor, also known as gain, describes the efficiency of a projection screen.
DIN 19045 describes the conditions for determining the luminous efficacy of a projection screen in comparison to a defined reference surface (white standard according to DIN 50339). This white reference surface is made of barium sulphate, a white chalk tile. With this white chalk tile as the reference surface, the luminance factor is 1. All information on luminance factors always refers to this reference surface. It is therefore possible that there are luminance factors greater than 1. In these cases, a projection surface reflects (front projection screen) or transmits (rear projection screen) more light than the white chalk tile.
The luminance is measured at different viewing angles. The luminance is highest when the projected light falls vertically onto the screen and the viewer is looking at the projection surface along the same axis.
In this case, the projection axis and the optical axis are the same and the viewing angle is 0°. The maximum luminance factor - gain - is measured here. However, the further away the viewer is from the projection axis, the larger the viewing angle and the smaller the luminance of the reflected light.
Assembly
The installation of the projection screens can be carried out by our trained and experienced installation staff.
Perforation and microperforation
The perforation or micro-perforation of a projection film serves to increase sound permeability in the medium and higher frequency ranges. At lower frequencies, sound passes through a closed PVC film almost unhindered.
With perforation, we are talking about approx. 57,000 holes per m² with an open area of approx. 7 %, with microperforation approx. 300,000 holes per m² with an open area of approx. 6 %.
If the viewer is relatively close to the projection screen, it makes sense to choose microperforation (e.g. GAMMALUX® MICRO); if the viewer is further away, a perforated projection screen (e.g. OPERA® perforated) can be chosen.
Attention: Depending on the projection screen distance and the type of projection, a so-called "moiré effect" may occur with digital projectors.
Embossing
Most projection screens have embossing on one side. The embossing has an optical function for the direction of the incident or transmitting light. This embossing should always point towards the viewer for both front and rear projection. The Gerriets HIGH GAIN film is an exception. With this film, the embossing must always be on the side facing away from the viewer, as the side facing the viewer is highly reflective. With ready-made projection screens, the front side can also be recognized by the Gerriets lettering on the eyelets.
Projection quality
Even when using high-quality projection screens, the projection quality depends on many different factors, such as the quality of the projector, ambient light and distance from the projection source. Particularly outdoors, projections in daylight are only possible under certain conditions.
PVC / Thermoplastics
Most projection films are made of PVC (polyvinyl chloride). PVC is an amorphous, thermoplastic material. It is hard and brittle, white in color and only becomes softer, malleable and usable for technical applications through the addition of plasticizers and stabilizers. The addition of further additives creates the conditions for flame retardancy.
Cleaning
To ensure a long service life, the films can be cleaned with a special PVC cleaning agent and a microfiber cloth. This cleaning agent must be solvent-free and must not attack the base material of the PVC film.
Repair
In order to be able to repair minor damage caused by external influences yourself on site, the films can be bonded with a special high-strength PVC adhesive and a replacement piece of the corresponding film. To avoid any misunderstandings: even minor repairs will always be visible, especially with rear projection films.
Reflectance, transmittance and absorptance
The reflectance represents the ratio of the reflected spectral radiant flux (luminous flux) to the incident spectral radiant flux. The transmittance shows the ratio of the transmitted to the incident spectral radiant flux and the absorptance represents the loss factor of a projection surface, i.e. the ratio between the absorbed and the incident spectral radiant flux. The value 1 (or 100 %) shown in a color spectrogram is made up of the reflection, transmission and absorption of the projection screen. The loss factor due to absorption is usually negligible, except in the case of cheap projection screens. The reflection or transmission factor is often confused with the luminance factor, but the measuring methods and the significance of the measured values are fundamentally different.
Rear projection
Projection that is projected onto the back of a screen. The following Gerriets projection screens are suitable for rear projection:
PANORAMA, OPTITRANS®, OPTILUX, TRANSMISSION, ARENA 86, ARCTIC
Black level
The black level represents the intensity of black in a projection on the projection surface. The black level is influenced by both the projection source and the projection surface. The optimum black level would theoretically be zero percent reflection or transmission of the incident light on the projection surface.
1 Text presentation with scattered light on white front projection screen
2 Text presentation with scattered light on GREYSCREEN film
Softedge projection
With softedge projection, media software must control the light intensity of the projectors at overlapping image edges to ensure an even, natural-looking projection with two or more projectors. However, the media software can only control the projectors; which medium or projection screen is being projected onto is beyond the software's control. When the luminance factor and luminance distribution were explained on the previous pages, the suitability of a projection screen for soft edge projections was also mentioned. If a projection screen with a high luminance factor and an unfavorable luminance distribution were selected for a softedge projection, it would have brighter areas in the respective 0-axis, i.e. the projection axis, with three projectors, for example, which would become darker towards the edge in order to become brighter again when entering the next projection axis. In other words, three lighter fields with darker edge areas. For softedge projections, it is essential to select projection screens with an optimum luminance distribution; here, the level of the luminance factor in the 0 axis is the secondary selection criterion. Projection films that fulfill this criterion in rear projection are, for example, OPERA® milky matt (ß 0.32), OPERA® light blue (ß 0.26), both with lower luminance factors but an almost ideal luminance distribution, and above all the new TRANSMISSION rear projection film, which successfully combines a high luminance factor (ß 1.13) with very good luminance distribution.
Scattering angle and half gain angle
The scattering angle of a screen results in the viewer area limitation according to DIN 19045. This viewer area limitation is defined as 40° to the left and right of the 0-axis or projection axis. The half gain angle only specifies the lateral angle to the projection axis at which 50% of the luminance factor ß still reaches the 0-axis and is rarely specified as an additional value to the luminance factor for the assessment of a projection screen.
Full screen projection
Projection of data or image material with a projector that completely illuminates the screen surface.
White quarry
Many amorphous plastics show a white coloration in stressed areas during stretching. This phenomenon is known as crazing. Crazing is a microscopically small area whose boundary surfaces are damaged with individual, extremely stretched or buckled material strands (crazes). Crazes are a pre-damage of the material and can be the beginning of a fracture.